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Effect of direct seeding mulch-based systems on soil carbon storage and macrofauna in Central Brazil

Blanchart E., Bernoux M., Sarda X., Siqueira Neto M., Cerri C.C., Piccolo M.D.C., Douzet J.M., Scopel E.. 2007. ACS. Agriculturae Conspectus Scientificus, 72 (1) : p. 81-87.

Soils represent a large carbon pool, approximately 1500 Gt, equivalent to almost three times the quantity stored in terrestrial biomass and twice the amount stored in the atmosphere. Any modification of land-use or land management can induce variations in soil carbon stocks, even in agricultural systems that are perceived to be in a steady state. These modifications also alter soil macrofauna that is known to affect soil carbon dynamics. Direct seeding Mulch-based Cropping (DMC) systems with two crops per year without soil tillage have widely been adopted over the last 10 to 15 years in the Cerrado (central region) of Brazil. They are replacing the traditional soybean monocropping with fallow under conventional tillage (CT). The objective of this study was to examine how DMC practices affect soil organic carbon (SOC) dynamics and macrofauna (Rio Verde, Goias State). The approach was to determine soil C stocks and macrofauna in five fields under DMC aged 1, 5, 7, 11 and 13 years. In order to compare DMC systems with the native system of the region and previous land-use, a situation under native Cerrado (tree-savanna like vegetation) and a field conducted traditionally (CT) were also studied. Soil C stocks were calculated for the 0-10 and 0-40 cm soil depth and also for the first 400 kg m-2 of soil to compare the same amount of soil and to suppress the potential artefact of soil compaction when sample is based on fix layer depth. Soil macrofauna was hand-sorted from soil monoliths (30 cm depth, TSBF method). In our study, the annual rate of carbon storage was equal to ca. 1.6 MgC ha-1, which is in the range of values measured for DMC in different areas of Brazil, i.e., 0.4 to 1.7 MgC ha-1 with the highest rates obtained in the Cerrado region. Compared to natural vegetation, soil macrofauna in cropped systems was strongly modified. In CT, biomass and density were very low and much lower than in DMC systems. With increasing age of DMC, total macrofauna density increased and then decreased while total macrofauna biomass continuously increased due to a strong increase in Coleoptera larvae biomass. These modifications in macrofauna density and biomass are discussed with regard to soil SOC dynamics (decomposition, mineralization and physical protection).
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